It is generally known that in order to obtain the optimum reinforcing properties imparted by a filler, this filler should be present in the elastomer matrix in a final form that is both as finely divided as possible and as uniformly distributed as possible. However, such conditions can be achieved only if the filler has a very good capacity, on the one hand, to be incorporated into the matrix during the mixing with the elastomer and to deagglomerate, and, on the other hand, to disperse uniformly in this matrix.
As is well known, carbon black exhibits such capacities, which is not generally the case for inorganic fillers. Indeed, for reasons of mutual affinities, the inorganic filler particles have an annoying tendency to agglomerate together in the elastomer matrix. These interactions have the harmful consequence of limiting the dispersion of the filler and thus of limiting the reinforcing properties to a level that is substantially below that which it would theoretically be possible to achieve if all the bonds (inorganic filler/elastomer) capable of being created during the compounding operation were actually obtained. Moreover, these interactions tend to increase the consistency in the uncured state of the rubber compositions, and thus to render the processing (“processability”) thereof more difficult than in the presence of carbon black.
Ever since savings in fuel and the need to protect the environment have become a priority, it has however proved necessary to produce tires that have a reduced rolling resistance without having a disadvantageous effect on their wear resistance. This has been made possible in particular by virtue of the discovery of novel rubber compositions reinforced with specific inorganic fillers that are described as “reinforcing” and that are capable of competing, from a reinforcing viewpoint, with a conventional tyre-grade carbon black, while giving these compositions a lower hysteresis, synonymous with a lower rolling resistance for the tires comprising them.
Such rubber compositions, comprising reinforcing inorganic fillers of the siliceous or aluminous type, have, for example, been described in the patents or patent applications EP-A-0501227 (or U.S. Pat. No. 5,227,425), EP-A-0735088 (or U.S. Pat. No. 5,852,099), EP-A-0810258 (or U.S. Pat. No. 5,900,449), EP-A-0881252, WO99/02590, WO99/02601, WO99/02602, WO99/28376, WO00/05300 and WO00/05301.
Mention will be made in particular of documents EP-A-0501227, EP-A-0735088 or EP-A-0881252 which disclose diene rubber compositions reinforced with highly dispersible precipitated silicas, such compositions making it possible to manufacture treads having a substantially improved rolling resistance, without adversely affecting the other properties, in particular the grip, endurance and wear resistance properties. Such compositions exhibiting such a compromise of contradictory properties are also described in applications EP-A-0810258 and WO99/28376, with, as reinforcing inorganic fillers, specific, highly dispersible aluminous fillers (aluminas or aluminium (oxide)hydroxides), or else in applications WO00/73372 and WO00/73373, which describe specific titanium oxides of reinforcing type.
The use of these specific, highly dispersible inorganic fillers, whether as the predominant reinforcing filler or not, has certainly reduced the difficulties in processing rubber compositions containing them, but this processing nevertheless remains more difficult than for rubber compositions conventionally filled with carbon black.
In particular, it is necessary to use a coupling agent, also referred to as a bonding agent, the role of which is to provide the bonding between the surface of the inorganic filler particles and the elastomer, while facilitating the dispersion of this inorganic filler within the elastomer matrix.
It is recalled here that the expression “coupling agent” (inorganic filler/elastomer coupling agent) is understood, in a known manner, to mean an agent capable of establishing a sufficient bond, of chemical and/or physical nature, between the inorganic filler and the diene elastomer; such a coupling agent, which is at least bifunctional, has, for example, a simplified general formula “Y—W—X”, in which:                Y represents a functional group (“Y” function) which is capable of bonding physically and/or chemically to the inorganic filler, such a bond possibly being established, for example, between a silicon atom of the coupling agent and the surface hydroxyl (OH) groups of the inorganic filler (for example, the surface silanols when it is silica);        X represents a functional group (“X” function) capable of bonding physically and/or chemically to the diene elastomer, for example via a sulphur atom; and        W represents a divalent group allowing Y to be linked to X.        
The coupling agents in particular must not be confused with simple agents for covering the inorganic filler which, in a known manner, may comprise the Y function that is active with respect to the inorganic filler but are devoid of the X function that is active with respect to the diene elastomer.
Coupling agents, in particular silica/diene elastomer coupling agents, have been described in a large number of documents, the most well known being bifunctional organosilanes bearing at least one alkoxyl function as the Y function, and, as the X function, at least one function capable of reacting with the diene elastomer such as for example a sulphurated (i.e., sulphur-containing) function.
Thus, it has been proposed in patent applications FR-A-2094859 or GB-A-1310379 to use a mercaptoalkoxysilane coupling agent for manufacturing tyre treads. It was rapidly demonstrated and it is today well known that mercaptoalkoxysilanes are capable of providing excellent silica/elastomer coupling properties, but that the industrial use of these coupling agents is not possible due to the very high reactivity of sulphurated functions of thiol —SH type (X functions) that very rapidly result, during the preparation of rubber compositions in an internal mixer, in premature vulcanizations also referred to as “scorching”, in very high viscosities in the uncured state, and ultimately in rubber compositions that are almost impossible to work and to process industrially. To illustrate this problem, mention may be made, for example, of documents FR-A-2206330, U.S. Pat. No. 3,873,489 and U.S. Pat. No. 4,002,594.
To overcome this drawback, it has been proposed to replace these mercaptoalkoxysilanes with alkoxysilane polysulphides, especially bis(alkoxylsilylpropyl)polysulphides as described in very many documents (see, for example, FR-A-2149339, FR-A-2206330, U.S. Pat. No. 3,842,111, U.S. Pat. No. 3,873,489, U.S. Pat. No. 3,997,581, EP-A-680997 or U.S. Pat. No. 5,650,457, EP-A-791622 or U.S. Pat. No. 5,733,963, DE-A-19951281 or EP-A-1043357 and WO00/53671). Among these polysulphides, mention should especially be made of bis(3-triethoxysilyl-propyl)tetrasulphide (abbreviated to TESPT) and bis(3-triethoxysilylpropyl)disulphide (abbreviated to TESPD).
These alkoxysilane polysulphides, in particular TESPT, are generally considered to be the products that provide, for vulcanizates comprising a reinforcing inorganic filler, in particular silica, the best compromise in terms of scorch safety, ease of processing and reinforcing power. They are, in this respect, the most widely used coupling agents today in rubber compositions for tires, even though they are relatively expensive and, furthermore, must most often be used in a relatively large amount.